Vacuum pumps are known which use a source of compressed air (or other high-pressure fluid) in order to generate a negative pressure or vacuum in a surrounding space. Compressed-air driven ejectors operate by accelerating the high pressure air through a drive nozzle and ejecting it as an air jet at high speed across a gap between the drive nozzle and an outlet flow passage or nozzle. Fluid medium in the surrounding space between the drive nozzle and outlet nozzle is entrained into the high-speed flow of compressed air, and the jet flow of entrained medium and air originating from the compressed-air source is ejected through the outlet nozzle. As the fluid in the space between the drive and outlet nozzles is ejected in this way, a negative pressure or vacuum is created in the volume surrounding the air jet which this fluid or medium previously occupied.
For any given compressed-air source (which may also be called the drive fluid), the nozzles in the vacuum ejector may be tailored either to produce a high-volume flow, but not to obtain as high a negative pressure (i.e., the absolute pressure will not fall as low), or to obtain a higher negative pressure (i.e., the absolute pressure will be lower), but without achieving as high a volume flow rate. As such, any individual pair of a drive nozzle and outlet nozzle will be tailored either towards producing a high-volume flow rate or achieving a high negative pressure.
A high negative pressure is desirable in order to generate the maximum pressure differential with ambient pressure, and so generate the maximum suction forces which can be applied by the negative pressure, for example for lifting applications. At the same time, a high-volume flow rate is necessary in order to ensure that a volume to be evacuated can be emptied sufficiently quickly to allow for repetitive actuation of the associated vacuum device, or equally in order to convey a sufficient volume of material, in vacuum conveyer applications.
In order to achieve both a high ultimate vacuum level and a high overall volume flow rate, so-called multi-stage ejectors have been devised, which comprise three or more nozzles arranged in series within a housing, each adjacent pair of nozzles in the series defining a respective stage across which a negative pressure is generated in the gap between the adjacent two nozzles. Again, in general, any individual pair of nozzles in the series may be tailored either towards producing a high-volume flow rate or achieving a high negative pressure, for a given source of compressed air.
In such multi-stage ejectors, the earliest stages produce the highest levels of negative pressure, i.e., the lowest absolute pressures, whilst the subsequent stages provide successively lower negative pressure levels, i.e., higher absolute pressures, but increase the overall volume throughput of the ejector device. In order to apply the generated vacuum across the multiple stages to a desired vacuum device or volume to be evacuated, the successive stages are typically connected to a common collection chamber, whilst valves are provided to each successive stage, at least after the first, drive stage, so that the subsequent stages can be closed off from the collection chamber once the negative pressure in that chamber has been reduced below the negative pressure which the second and subsequent stages are able to generate.
The drive stage is so-called because it is the only stage connected to the source of pressurised fluid (compressed air), and so drives the flow of pressurised fluid through all of the subsequent stages and nozzles in the series, before the drive fluid and entrained fluid is ejected from the vacuum ejector.
In order to provide for the entrainment of fluid across each successive stage, the series of nozzles present a through-channel with gradually increasing sectional opening area, through which the stream of high-speed fluid is fed in order to entrain air or other medium in the surrounding volume into the high-speed jet flow. The nozzles between each stage form the outlet nozzle of one stage and the inlet nozzle of the next stage, and are configured to successively accelerate the flow of air and other medium in order to direct a high-speed jet of the fluid across each successive stage.
Although different pressurised fluids may be utilised as the drive fluid, multi-stage ejectors of the present type are typically driven by compressed air, and most usually are used to entrain air as the medium to be evacuated from the volume surrounding the jet flow through each gap in the series of nozzles, across the respective stages.
One design of multi-stage ejector which has found commercial success is to present the series of nozzles in a coaxial arrangement within a substantially cylindrical housing which incorporates a series of suction ports therein in communication with each stage of the ejector, the suction ports being provided with suitable valve members for selectively communicating each stage with a surrounding volume of air. So presented, the cylindrical body is formed as a so-called ejector cartridge, which, when installed inside a housing module, or within a suitably dimensioned bore hole, can be used to evacuate the surrounding chamber, which is in turn fluidly coupled to the vacuum device to which the negative pressure is to be applied.
Such a device is disclosed in PCT International Publication No. WO 99/49216 A1, in the name of PIAB AB, and is shown in FIGS. 14 and 15 of the present application.
As shown in FIG. 14, the ejector cartridge 1 comprises four jet-shaped nozzles 2, 3, 4 and 5 which define a through-channel 6 with gradually increasing cross-sectional opening area. The nozzles are arranged end-to-end in series with respective slots 7, 8 and 9 between them.
The nozzles 2, 3, 4 and 5 are formed in respective nozzle bodies, which are designed to be assembled together to form an integrated nozzle body 1. Through openings 10 are arranged in the wall of the nozzle body, to provide flow communication with an outer surrounding space.
Turning to FIG. 15, it can be seen how the ejector cartridge 1 may be mounted within a bore hole or housing, in which the outer surrounding space corresponds to a chamber V to be evacuated. Each of the through openings 10 is provided with a valve member 11 in order to selectively permit the flow of air or other fluid from the surrounding space V into the space or chamber between each adjacent pair of nozzles. As shown in FIG. 15, the ejector cartridge 1 has been mounted in a machine component 20, in which the bore hole has been drilled or otherwise formed. The ejector cartridge 1 extends from an inlet chamber i to an outlet chamber u, and is arranged to evacuate the three separate chambers constituting the outer surrounding space V, each of which is separated from the adjacent chamber by an O-ring 22. Although not shown, each of the chambers constituting the outer surrounding space V is connected to a common collection chamber or suction port, in order to apply the generated negative pressure to an associated vacuum-operated device, such as a suction cup.
Although such multi-stage ejector arrangements are beneficial in providing both a high-volume flow rate and a high level of negative pressure, there is necessarily still some degree of compromise in the design of each successive stage in the ejector, in order to obtain an overall desired performance characteristic for the multi-stage ejector as a whole. Accordingly, it has also been proposed to provide a further so-called booster nozzle, provided in parallel with the drive nozzle of the multi-stage ejector, where the booster nozzle is specifically designed to obtain the highest possible level of vacuum, but does not form part of the series of coaxially arranged nozzles which make up the multi-stage ejector. In this way, the booster nozzle can be configured to obtain the highest possible level of vacuum, whilst the parallel multi-stage ejector nozzle series can be arranged to obtain a high-volume throughput, which enables a high negative pressure (low absolute pressure) to be obtained within the volume to be evacuated within an acceptably short period of time.
Such an arrangement is disclosed in U.S. Pat. No. 4,395,202, as shown in FIG. 13 of the present application. In this arrangement, there is provided a set of ejector nozzles 12, 13, 14, 15 arranged successively for evacuation of associated chambers 5, 6, 7, which are in mutual communication with a vacuum collecting compartment 16 through respective ports 18, 19 and 20. Valves, 21, 22 and 23 are respectively provided to the ports 18, 19 and 20.
An additional pair of nozzles 24 and 25 is provided in parallel to the drive nozzle 12 of the multi-stage ejector, and is arranged in a separate booster chamber 4, connected to the collecting chamber 16 via a port 17. The booster stage is comprised of a pair of nozzles 24 and 25, with the inlet nozzle 24 being connected, together with the drive nozzle 12 of the multi-stage ejector, to the inlet chamber 3, which is supplied with compressed air. The pair of nozzles 24 and 25 across the booster stage serves to generate the highest possible vacuum (lowest negative pressure) in the booster chamber 4. The jet of compressed air which is generated by the nozzle 24 is ejected out of the booster stage through nozzle 25, into the same chamber 5 across which the drive nozzle 12 propels the drive jet of compressed air. In this way, the air expelled out of the booster stage is entrained into the drive jet flow to be expelled from the multi-stage ejector. Furthermore, the vacuum generated by the drive stage of the multi-stage ejector is applied to the exit of nozzle 25, so that the pressure differential across the booster stage is increased whereby the vacuum level which can be generated by the booster stage can be increased, i.e., the absolute pressure which can be obtained is reduced.
In operation of the vacuum ejector, the series of nozzles 12, 13, 14 and 15 of the multi-stage ejector is able to produce a high volume flow rate so as quickly to generate a vacuum to a low absolute pressure in the collecting chamber 16 within a short period of time by entraining fluid from each of the chambers 5, 6 and 7 and the collecting chamber 16 into the jet streams formed by each successive stage of the ejector. The booster stage functions in parallel to the multi-stage ejector, but typically produces a low volume flow rate, and so does not contribute significantly to the initial vacuum formation process. As the vacuum level in the collecting chamber 16 increases (i.e., as the absolute pressure falls), the associated valve members 23, 22 and 21 will close in turn, as the pressure in the vacuum collecting chamber 16 drops below the pressure in the associated chamber 7, 6 or 5, respectively. Eventually, the pressure in the collection chamber 16 will fall below the lowest pressure that any of the stages of the multi-stage ejector is able to generate, so that all of the valves are closed, and all further evacuation will then be done by the booster stage, which provides suction to the collection chamber 16 via suction port 17.
Such multi-stage ejectors and ejector cartridges as described above have found commercial success in a number of different industries, and in particular in the manufacturing industry, where such vacuum ejectors may be connected to suction cups and used for picking and placing components during an assembly process.
As the demands for high vacuum levels (i.e. low absolute pressures) in processes such as de-gassing, de-humidifying, filling of hydraulic systems, forced filtration, etc., continue to increase, there is increasing demand for vacuum ejectors which are able to repeatedly provide a high level of negative pressure (i.e., a low absolute pressure) in order to carry out the above and other processes.
Coupled with this, there is an increasing drive towards smaller-sized ejectors, which are able to provide the desired evacuation capability at remote locations on the machinery (i.e., at the ends of mechanical arms, and significant distances from the ultimate source of compressed air) without negatively impacting on the overall dimensions of the machine. In particular, there is a desire for ejector devices having a small footprint, and so able to apply a vacuum to increasingly compact working areas.